High temperature oxidation resistant alloy

ABSTRACT

A low-cost ferrous alloy having improved high-temperature oxidation-resistance under conditions of cyclic heating at temperatures up to about 2500*F, and consisting essentially of, by weight percent, about 4.0 to 8.2% aluminum, up to about 10.5% chromium, about .05 to 2.0% titanium, with the balance iron and incidental impurities, said alloy further characterized by an f factor &gt; OR = 9.0, where:

United States Patent 1191 1111 3,859,079

Giles et al. 1 Jan. 7, 1975 [54] HIGH TEMPERATURE OXIDATION 2,941,883 6/1960 lda 75/124 IS N ALLOY 2,987,394 6/1961 Mueller 3,068,094 12/1962 Zackay Inventors: Phlllp M. Glles; Arnold r 3,676,109 7/1972 Cooper 75/124 both of Bethlehem, Pa. [73] Assignee: Bethlehem Steel Corporation, Primary Examiner H yland Bizot Bethlehem Attgrnfy, Agent, or F1rm.loseph .1. O Keefe; W1ll1am B. o [22] Filed: Aug. 9, 1972 [21] Appl. No.: 275,650 ABSTRACT A low-cost ferrous alloy having improved hightemperature oxidation-resistance under conditions of 75/124C,2725c/13276/ll()) cyclic heating at temperatures up to about 2500,13 and Consisting essentially of by weight percent, about [58] Field of Search 75/124, 126 D 4.0 to 82% aluminum up to about 10.5% Chromium [56] References Cited about .05 to 2.0% titanium, with the balance iron and incidental impurities, said alloy further characterized UNITED STATES PATENTS by an ffactor 2 9.0, where: 1,621,523 3/1927 Clement 75/124 V 1,641,752 9/1927 Flintermahn 75/124 f(% 041% Cr) 1,833,723 11/1931 Ruder 75/124 3 Claims, lDrawing Figure HIGH TEMPERATURE OXIDATION RESISTANT ALLOY BACKGROUND OF THE INVENTION This invention relates to a low-cost, hightemperature oxidation-resistant ferrous alloy suitable for applications where thermal shock, such as cyclic heating and cooling, are experienced. While not intending to unduly limit this invention, applications which can take advantage of the properties hereof are high temperature exhaust systems in automobiles, jet engines, and in the petrochemical industry.

Heretofore, high-temperature oxidation-resistant materials were selected from high cost nickel and cobalt superalloys or ceramic materials. In an effort to lower costs, the prior art moved to lower or less rich alloys. For example, U.S. Pat. No. 1,641,752 teaches a ferrous alloy resistant to oxidation at high temperatures, by including therein a high percentage of aluminum. Specifically, said alloy contains'from 12 to 20% aluminum, and about 1 to of a grain refining material, among which the patentee has included titanium and chromium. The high temperature oxidationresistance is due at least in part to the formation of a protective coating of oxide of the aluminum on the exposed surfaces of the ferrous alloy. However, such alloys are of limited suitability under cyclic heating and cooling conditions wherein thermal shock results in flaking and spalling of the oxide coating. Accordingly, one of the critical requirements of a suitable alloy is its ability to resist such flaking and spalling.

SUMMARY OF THE INVENTION The present invention relates to a ferrous alloy which is not only resistant to oxidation at high temperatures, but is resistant to surface flaking and spalling when subjected to thermal shocking as a result of cyclic heating and cooling. More particularly, said invention covers an element balanced alloy consisting essentially of, by weight percent, about 4.0 to 8.2% aluminum, up to about 10.5% chromium, about 0.05 to 2.0% titanium, with the balance iron and incidental impurities. Further, the element balancing is achieved by the formula: f=(% Al) 0.4(% Cr) 2.5(% Ti), where the value offis at least 9.0

BRIEF DESCRIPTION OF DRAWINGS The FIGURE is a graph of time of exposure to failure vs.ffor,a series of ferrous alloys falling within the alloying ranges noted previously, with time plotted on a logarithmic scale.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT This invention relates to an oxidation resistant ferrous alloy, more particularly to a wrought ferrous alloy suitable for use in applications where ability to withstand thermal shock is critical. Typical applications where the latter characteristic is significant are high temperature exhaust systems, such as a thermal reactor in automobiles, jet engines, and in the petrochemical industry. While the mention of same is not intended as a limitation on this invention, it is believed that a brief discussion thereofwill help in understanding the significance of this invention and the contributions offered thereby.

Generally, a thermal reactor is a container into which the hot exhaust gases flow from the automotive, jet or power producing engines, for further combustion. Air is also pumped into the reactor and admixed with said gases. The reactor is generally of sufficient size (chamber volume) to give a long enough residence time ofthe admixed gases to permit complete combustion of the residual hydrocarbons and carbon monoxide. Since the combustion reaction is so strongly exothermic, temper atures may go as high as 2200F., typically about 1800F., in a highly oxidizing environment. Thus, the severe conditions of high temperature, oxidizing environment, and intermittent operation (cyclic heating and cooling), Call for a material capable of meeting these conditions. The alloys of the present invention not only fulfill these conditions, but accomplish it at a low material cost.

The composition of the alloys of this invention fall broadly within the alloying ranges below:

Aluminum 4.0 to 8.2 wt. Titanium .OS to 2.0 wt. Chromium up to 10.5 wt. Carbon up to 0.10 wt. Iron balance,

except that within said ranges the composition must satisfy the formula,f=(% Al) 0.4(% Cr) 2.5(% Ti) B 9, and the titanium content must exceed 4 times the carbon content. A preferred composition is one containing at least 5.0% chromium, aluminum between about 6.0 to 8.0% and titanium between about 0.4 to 1.10%.

While no maximum value forfhas been noted, if desirable one can readily be calculated from the richest composition contained within the limits above. However, it is believed that such a figure would not be significant as to the alloys performance under severe oxidizing conditions. The maximum composition limits, and hence the maximum f, are established by other considerations, such as material cost and processing restrictions.

In this regard, the production of wrought strip and sheet by conventional rolling practices requires that the material be sufficiently ductile for said rolling to be effected. In the case of aluminum containing iron base alloys, the ductility thereof drops below a practical level when the aluminum begins to exceed about 8% by weight. Chromium, when added to an Al-Fe alloy, should not exceed about 10% by weight, as the risk of breakage and other processing problems increases. Finally, with respect to carbon it should be kept to a low value, preferably less than about 0.04% typically less than 0.03%. But in no case should it exceed 25% of the titanium.

It should be apparent by this time that the significant features of this invention are achieved by a proper balance of the alloys composition such that the value of f, heretofore defined, exceeds about 9.0. With such an alloy, it will be observed that upon exposing said alloy to a highly oxidizing atmosphereat high temperatures, there develops an adherent oxide coating. This coating is predominantly oxides of aluminum, titanium and combinations thereof, with little or no iron oxide. This iron oxide free coating resists flaking, spalling and thermal shock so as to protect the alloy from catastrophic oxidation. As used herein, catastrophic oxidation is the presence of black oxide coating in an amount greater than 30% of the surface area exposed atmosphere. 1

The very adherent oxide coating which forms on the alloy of this invention must contain some oxides of titanium. Thus, while it may not be apparent at this juncto the oxidizing The samples were periodically inspected and considered to have failed when either about 30% of the sample surface was covered with a porous black oxide, or when the area around the hole oxidized to the point of v 5 actually failing or by their appearance being unable to ture, the titanium forms a dual function. The carbon in support the weight any further. an alloy of the type described herein tends to migrate In any case, by these objective standards, it is readily to the grain boundaries as carbides. By adding titanium, apparent from the tabular and graphic data that a sig- 8 t g Carbide former, the Carbon is readily e P nificant improvement resulted when the composition of as titanium carbide and is uniformly dispersed through- 10 the alloy was balanced to anfvalue of about 9.0, and out the alloy. Thus, sufficient tltanlum must be Present titanium was present in an amount of at least, 0.05%. to handle the carbon as well as to be free to assist in Note specifically samples Q and R where fexceeded r g the adhere"t Oxide coating which resistfi P 9.0 but the titanium was'lacking. A comparison of Samlhg and flakihg- The slghlfieahce 0f the mahlum 1S ples Q and T, whose difference is essentially the addiclearly illustrated 1n the data to follow. tion of about 0.49% Ti, shows nearly a thirteen (13) To demonstrate the effectiveness of the ferrous alloys fold improvement in oxidation resistance with said titaof this invention to resist oxidation at high temperature, nium. attention is directed to the following table and accom- We claim: panying FIGURE, which dramatically illustrate the sul. A ferrous alloy resistant to catastrophic oxidation perior results achieved in balancing the composition under conditions of repeated cyclic heating and cooling thereof, so as to reflect a value for f of at least 9.0. between the temperatures of 2,200 F. and 200 F., said TABLE Time-to-Failure (Hours) at Sample C Al Cr Ti f 2200 F.

A .009 4.20 4.20 .5 B .015 4.04 4.67 .5 C .012 6.00 6.00 23 D .004 4.11 5.03 6.l3 23 E .003 4.07 4.97 .28 6.76 23 F .015 4.02 1.10 6.77 .5 G .020 5.02 4.96 7.00 23 H .016 5.98 .54 7.33 23 J .019 4.93 4.94 .25 7.52 47 K .022 4.01 9.66 7.87 23 L .009 8.12 8.12 63 M .008 8.10 .28 8.80 1087 N .013 8.00 .52 9.30 1771 P .037 6.92 3.08 .53 9.46 1565 Q .016 5.97 9.74 9. 7 111 R .019 7.92 5.00 9.92 229 s .015 8.01 1.02 10.56 2631 T .016 5.92 9.75 .49 11.04 2998 u .007 6.18 10.32 .53 11.63 I 3300 where: l'= wt) Al .4 wt) Cr 2.5 wt) Ti. Mo, P. S, Sn .0l% Cu, Mn, V .02% Ni .03%

The procedure used to determine time-to failure at ferrous alloy composed of, by weight percent, a maxi- 2200F. was as follows: mum of 0.04% carbon, aluminum between about 4.0

The samples nominally 1 inch X 2 inches X 0050 and 8.2%, chromium up to about 10.5%, titanium beinch, were hung on silicon carbide bars using chromel .tween and 119%; and the balance substimtlany wire through a hole drilled in one end thereof. The bars except that Sam ferrous alloy charactenzed by were then placed in an exposed-element, electrically a Value off of at least about whe;re: heated furnace at 2200 i 20 F. Twice daily, the silif (%Al) 04(%Cr) and (%T') con carbide bars with the samples attached were re- 4(%C) moved from the furnace where the samples were alh alloy cialmed m clam 1 wherem Sald lowed to air cool in still air for about 20 minutes at mlum 18 present 3 an of at which time the temperature had dropped to about T alloy clalmed m clam l i' Said alum! 200F. The samples were then returned to the furnace. g g; is present m an amount between about and The long term average cycle time was about vl5 hours. I 

1. A FERROUS ALLOY RESISTANT TO CATASTROPHIC OXIDATION UNDER CONDITIONS OF REPEATED CYCLIC HEATING AND COOLING BETWEEN THE TEMPERATURES OF 2,200*F. AND 200*F., SAID FERROUS ALLOY COMPOSED OF, BY WEIGHT PERCENT, A MAXIMUM OF 0.04% CARBON, ALUMINUM BETWEEN ABOUT 4.0 AND 8.2%, CHROMIUM UP TO ABOUT 10.5%, TITANIUM BETWEEN 0.4 AND 1.10%; AND THE BALANCE SUBSTANTIALLY IRON, EXCEPT THAT SAID FERROUS ALLOY IS CHARACTERIZED BY A VALUE OF F OF AT LEAST ABOUT 9.0 WHERE: F = (%A1) + 0.4(%CR) + 2.5(%TI), AND (%TI) > 4(%C).
 2. The alloy claimed in claim 1 wherein said chromium is present in an amount of at least 5.0%.
 3. The alloy claimed in claim 1 wherein said aluminum is present in an amount between about 6.0 and 8.0%. 